This invention relates to a drill bit for use in a drill string comprising a plurality of central rod members for transmitting impacts to the drill bit and a plurality of tubular members for transmitting rotation to the drill bit, said tubular members surrounding said rod members, wherein the drill bit is provided with a flushing channel which extends rearwards from the bit front and terminates in an intermediate portion of the drill bit, and a transverse passage at the intermediate portion, said transverse passage interconnecting said flushing channel and an annular space formed between the rod members and the tubular members.
In a drill string of the above-described type the central rod members are adapted to transmit impact energy and feeding force to the drill bit, and the tubular members are adapted to transmit torque and withdrawal force to the drill bit, whereby an extension rock drill equipment is obtained in which the energy losses are small. A drill string of this type is described in U.S. Pat. No. 4,094,364 which is incorporated in the present description by way of reference.
In the device described in U.S. Pat. No. 4,094,364 the drill bit comprises a main body provided with cutting means which is threadedly connected to the elongated shank portion of the drill bit, said shank portion being inserted in the forward end of the tubular members. In this two-piece design of the drill bit, the bit is guided comparatively far away from the bit front, which means that the hole might have a tendency to deviate from the desired drilling direction. Further, the threaded connection between the main body and the shank portion means that the use of threads is not completely eliminated; ie threads causing energy losses and threads that might be broken.
The object of the invention is therefore to provide a drill bit of the type in question in which no energy losses do arise in the drill bit itself, and which is guided as close as possible to the bit front.
For strength reasons it has been found necessary that a drill bit of the type in question is closed at its rear impact-receiving end surface. Further, flushing fluid must be conducted to at least two spaced positions on the bit front in order to render the flushing efficient. Therefore, another object of the invention is to provide a drill bit in which the flushing channel is produced and designed in such a way that the above-mentioned two requirements are met.